There are a variety of known types of tubing formed of insulating material which may be used in many applications where good insulation and similar properties are required. It has been found that tubing formed of foam plastic material such as polyurethane material is particularly effective as an insulating material.
It is known that continuous lengths of tubing of the aforesaid type may be formed by utilizing a forming mold constructed of upper and lower semi-cylindrical mold elements arranged on continuously movable conveyors so that the upper and lower mold elements come into contact with one another to form a moving cylindrical mold. A mandrel is disposed centrally of the cylindrical mold, and flexible strips of material are formed into cylinders which cover the exterior surface of the mandrel and the interior surface of the cylindrical mold. A liquid forming material is injected into the annular space between the strips of flexible material to be cured during its passage through the moving cylindrical mold, or during its passage through a curing oven if required. The method and apparatus for forming tubing of the aforesaid type are disclosed in greater detail in U.S. Pat. Nos. 3,745,621; 3,746,487; 3,754,064 and 3,816,043.
While tubing formed by this known process is commercially acceptable, it will be appreciated that the moving molding cylinder arrangement is complex in design and operation, and it is expensive to produce and run on a continuous basis. Additionally, it is somewhat difficult to assure proper curing of the plastic mixture during the time it is within the moving molding cylinder.
In accordance with the present invention, a method and apparatus for forming a continuous length of tubing having foamed plastic walls is provided which obviates the disadvantages of known technology, and which provides substantial general improvements to such technology.